Efficient mixing of fluids and solids is essential for many industry sectors. The means by which this mixing is undertaken are many, the choice of which is dependent upon the nature of the materials being mixed and the degree and rate of mixing required.
Numerous concepts and frequent efforts have been made to improve the efficiency and effectiveness of liquid and solid mixing systems. Several notable methods that have met with relative success, depending upon the nature of the materials being mixed, have included: nozzle geometry distortion, motive flow pulsation, and the introduction of a diffuser as part of the system.
Nozzle distortion attempts to create turbulent flow by altering the geometry of the interaction of the motive flow with the nozzle surface, as shown in FIGS. 1a and 1b. The result of such an alteration is to change the velocity of the motive fluid as it exits the outlet of the nozzle creating vortices in which liquid-liquid or liquid-solid mixing can occur. Referring to FIG. 2a, typical geometries generate a narrow circular or near circular jet 300 that minimizes solids entrainment, hence minimizing the mixing effectiveness of liquid-liquid or liquid-solid vortices. As shown in FIGS. 2a-d, nozzle distortions 300 will quickly decay and eventually return to a circular or near circular shape. In addition, when solids 310 are introduced from the top by gravity into a larger cavity containing the liquid jet stream 300, only a small portion of the solids make contact with the liquid.
Referring to FIG. 3, a fluid velocity profile is shown for a prior art nozzle. The liquid jet stream 300 emanating from the initial mixing chamber reaches an upper range of 53.6 to 67.0 ft/sec, depicted as reference 320. As can be seen, this high velocity pierces through the solids that are introduced from above. Slower fluid velocities in the range of 40.2 to 53.6 ft/sec are depicted as reference 322 and are present ahead of the higher velocity stream 320 and in a boundary layer around stream 320. The fluid velocity slows even more downstream to a range of 26.8 to 40.2 ft/sec as depicted by reference 324. Upon entrance to the constricted area 312, and the diverging area 314, the velocity is slower, in the range of 13.4 to 26.8 ft/sec, shown by reference 326. It is in this entrance to the constricted area 312 that the velocity profile shows a single mixing zone 330. The slowest velocity, 0.00 to 13.4 ft/sec, shown by reference 328, is present along the edges of diverging area 314 as well as in initial mixing chamber where solids 310 are added at an angle normal to, or nearly normal to, the direction of fluid through the nozzle.
In motive flow pulsation, pulsating the velocity of the motive flow, either with or without a nozzle, does change the velocity that creates turbulent flow, but will not permit the maintenance of a vacuum conducive to consistent and rapid induction of the secondary solid. Furthermore, such efforts require additional control systems and external energy reducing the efficiency of the process.
A third methodology which has seen more positive results is that of the motive flow utilizing the combination of nozzle and diffuser. This combination is referred to as an eductor. The relative velocity of the motive flow passing through the void on the outlet of the nozzle effectively maintains the vacuum required to permit induction of the secondary solids, but does not create recirculation zones sufficient in size and intensity to permit optimal mixing.
The action of the motive flow through the nozzle into the void space at the outlet of the nozzle carries the secondary solid into the eductor but does not succeed in mixing the two to any great extent. All nozzle geometries create vortices at the micro level downstream of the nozzle. It has been suggested that some nozzle geometries, such as lobed nozzles, can create these vortices faster (i.e. at a lower pipe diameter lengths) for liquid in liquid applications. However, the intensity of the vortices does not change and applications to induced solids in liquid are unknown._ Furthermore the speed at which the micro vortices are created in eductor based liquid-solid mixing applications is not critical as several pipe diameters are available prior to discharge.
The creation of a vacuum to induce solids into the motive fluid and large eddy current vortices is necessary to entrain and mix the solids with the motive fluid. Therefore, without the addition of a downstream diffuser which is used to create vacuum and create short and intense large eddies, mixing is limited and solids are simply carried along the plane of the motive flow only to be inefficiently mixed several pipe diameters downstream at a very slow rate.
One effective method of controlling the location of large eddies and recirculation mixing zones created between the nozzle outlet and the diffuser inlet is through nozzle and diffuser geometry and position. Through the combination of these geometries and positions, several large eddies are generated that maximize solids induction and solid-liquid interface while limiting pressure drop. Typically, nozzles with or without distorted geometries are placed in the center of the motive flow and produce only limited contact with the solids and motive fluid. Therefore the turbulence and consequent mixing along the linear axis of the motive flow are limited. Further, protruding nozzles can be an impediment to the induction of the solids. Such an impediment will reduce the induction rate and negatively impact mixing performance.
This problem has been addressed with the introduction of a multi-lobed circular nozzle in conjunction with a lightly tapered single throat diffuser. While effective, this concept can be improved upon in such a manner so as to increase the rate at which secondary solids can be induced into the motive flow, improving the solids-liquid surface contact through a flat profile jet stream, improve the generation of three large eddy currents through the use of diffuser geometry, maintain turbulent flow throughout the mixing body through nozzle and diffuser geometry, increase and maintain the vacuum which facilitates the rapid induction of solids, reduce the pressure loss through the eductor system through nozzle geometry and improve overall mixing performance as measured by rate of hydration of secondary solids.